The invention relates to an impregnated sintered material for electrical contact members, consisting of a silver and/or copper filled skelton of tungsten, silver and/or copper, and an additive of a wetting promoting metal such as iron, nickel, cobalt, and to a method for its production.
Because of its high thermal and electric conductivity silver is a preferred material for contact members. However, in an arc such as those originating from the breaking and making of contacts, the silver located in the contact area of such contact members evaporates and spatters easily. This leads to high material wear (burning). Moreover, particularly when chatter occurs in contact making, silver contact members tend to freeze and fuse readily so that they can be reseparated from each other only by using force (known as the fusing force). Therefore, particularly in low voltage switch gear, contact members which contain graphite particles embedded in the silver, whereby the fusing force is reduced considerably, are usually used. Such contact members also have only a slight electrical contact resistance. Since graphite is not wettable by liquid silver, such contact members are usually produced by pressing and sintering a mixture of silver and graphite powders at temperatures below the melting point of silver. The porosity of sintered materials produced in this manner is high, usually at least 5 to 7 percent by volume, and their mechanical stability is poor. Furthermore, their thermal and electrical conductivity is reduced due to the embedded graphite. As a result, the burn-off losses of such sintered materials are high. This applies also to sintered materials consisting of copper with embedded graphite.
Compared to silver and copper, the mechanical hardness and burning resistance of tungsten is greater. Therefore, when tungsten powder is compacted and subsequently impregnated with liquid silver or copper at temperatures above their melting points, an impregnated material consisting of a tungsten skeleton which is filled with silver or copper is obtained. Since tungsten is wetted poorly by silver, it is advantageous to admix small amounts of iron, nickel, or cobalt to the silver, whereby the solubility of tungsten in the molten silver and its wettability are improved. Compared to tungsten, tungsten carbide, which forms from a mixture of tungsten and graphite powders at high temperatures, has an even greater hardness and burning resistance. According to H. Schreiner, "Pulvermetallurgie elektrischer Kontakte" (Powder Metallurgy of electrical contacts), Berlin/Gottingen/Heidelberg, 1964, ages 148/9, an impregnated sintered material can be produced by first producing a sintered skeleton of a powder containing tungsten, copper or silver, and nickel, and subsequently impregnating this skeleton. Such a material contains practically no pores, and its hardness, burning resistance and conductivity are improved. But, these advantages are obtained at the expense of a greater fusing force and of a usually higher electrical resistance.